The most efficient way of molding double-walled boxes has been to blow mold them from a single parison formed laterally into a molding space between a cavity and core. This limits the depth of such boxes to one-half their width. Greater depths for blow molded double-walled boxes have long been sought, and the need for this has produced several suggestions. Nagai U.S. Pat. No. 3,636,151 suggests two parisons arranged side by side and blown into box halves joined together along a seam line; and U.S. patent applications Ser. Nos. 827,865 and 828,253, both abandoned, assigned to the assignee of this application, suggest a way of blow molding deep, double-walled boxes in two parts that are later joined together. In both of these solutions, the production measures relating to the seam lines add to the expense of the boxes.
We have discovered a way that a deep, double-walled box can be blow molded from a single parison that is closed and draped downward over a core so that a leading portion, next to the core, becomes an inside wall of the box; and a succeeding portion, surrounding the core, becomes an outside wall of the box. Outer mold parts then close around the parison draped over the core, and the parison is blown in the cavity between the core and the outer mold parts to form a double-walled box that is unlimited in depth.
Our invention seeks economical, effective, and reliable blow molding of double-walled boxes that are unlimited in depth and are blown in a single piece from a single parison. Our way of accomplishing this includes molding methods and equipment and results in a uniquely formed, double-walled box. Although our way of making deep, double-walled boxes was to satisfy a need for blow molded boxes having interwall spaces that can be filled with a concrete type of fire resistant insulation material, our invention is not limited to such uses and can be applied to deep, double-walled boxes made for other purposes, such as thermally insulated ice chests, for example. Also, although this application refers to the blown product as a "box", it need not have a box shape or be rectangular in cross section; and it can have a circular, elliptical, or other cross section for a shape that might not ordinarily be called a "box". Regardless of the shape, however, the blown product is double walled, relatively deep from its bottom to its open end, and formed of a sinle parison draped over a core.